Many types of equipment and processes require apparatus to control their operation so that they perform steps in predetermined sequences. To effect the control of these steps various types of controllers have been used, ranging from simple mechanical devices up to microprocessor driven process control systems. Where electronic control is required, a microprocessor based system is very capable, providing for programmable control sequences of any desired complexity. However, where the control sequences are comparatively simple, the cost of the microprocessor and the ancillary equipment needed for its operation can be excessively expensive for the application. Further, the flexible control of a microprocessor requires that it execute a number of operational cycles to effect the steps in a control sequence, thus rendering the microprocessor slower, as well as more expensive, than is desirable.
What is needed is a sequence controller that is simpler in construction and operation, and therefore less expensive than the conventional microprocessor based controllers, but which have the capability of commanding any of several sets of operational sequences.